The invention is particularly of advantage in applications where the depletion of pressurized gas supply is critical and could result in physical danger of suffocation or harmful operation of gas utilizing equipment. The example used in this description is an oxygen gas resuscitator/transport ventilator where an oxygen gas is provided from pressurized gas cylinders or a pipeline through the transport ventilator to a patient face mask. Typically such transport ventilators are used in trauma situations by hospital and ambulance crews, firefighters, military medics, and miners during life threatening emergencies. However, it will be understood that the invention may be applied to any gas utilizing device where warning of low supply pressure is required, and especially where volatile gases are present and electric powered alarms are therefore undesirable.
Basic components of a transport ventilator system include a hand held mask fitted over the patient's nose and mouth, with an automatic resuscitator/ventilator and pressure regulator to control breathable gas flow from a high pressure source of pure oxygen gas.
In the relevant field of cardiopulmonary resuscitation and ventilation, patients during transport and at the scene of an incident are treated with pneumatic or electro-pneumatic ventilators to maintain the patient's respiration.
During this critical period, the operator's attention is primarily focused on the patient and on performing the tasks necessary to maintain the patient's life. Timing of activities is extremely crucial since lack of oxygen can cause permanent damage or death in minutes. The operator has usually taken some time to arrive at the scene and to access the required equipment. In such an environment, ventilation equipment that operates reliably is absolutely essential and automatic operation is highly desirable.
Gas supply pipelines and supply cylinders are normally fitted with pressure regulators, filters and a visual pressure gauge to indicate the pressure of cylinder contents or incoming pipeline pressure. The operator may not be aware of nor have immediate access to the visual pressure gauge.
The ventilator/resuscitator and regulating devices provide a reducing output performance, in terms of pressure and flow delivered to the patient face mask, as the supply diminishes. In the case of pressurized gas cylinders, supply diminishes as the contents are expelled, and where pipelines are used in hospitals for example, gas supply may be interrupted due to supply system malfunction.
Experience in practical life threatening situations has brought into question the prudence of relying on regulator pressure gauges and operator attentiveness, especially in life-threatening situations where operators are highly stressed and busy with other critical matters.
Some prior art regulators are notoriously inaccurate in terms of output performance. For example, regulators generally perform well maintaining constant pressure when gas is passed through the regulator at relatively low flow rates. However at high flow rates, some conventional regulators cannot maintain constant pressure as flow load increases.
A typical regulator usually includes an intake pressure or cylinder contents gauge, however, many regulators do not include an output pressure gauge. Without an output pressure gauge the functioning of the regulator is not monitored. No indication is given of whether gas is actually passing through the regulator, nor whether the regulator is impeding gas flow to such a high degree that malfunction of the regulator is indicated.
Many conventional regulators on the market operate relatively well when gas supply cylinders are at least 3/4 full but a reliable indication of contents is not provided when the cylinder contents deplete further. To address the inaccuracy of conventional regulators, manufacturers of ventilators/resuscitators often include a second more accurate regulator, pressure sensors and gauges built into their equipment.
This serious problem with conventional breathable gas supply methods has been recognized by the relevant regulatory agencies. Recent standards and proposed standards for such equipment include a requirement for an audible power failure alarm to indicate loss of electrical or pneumatic driving power.
For example, International Standard ISO 10651-3, 1st ed. Jan. 15, 1997, Lung Ventilators for Medical Use--Part 3: Particular requirements for emergency and transport ventilators, Section 8.2.51.5.1 headed: Power Failure Alarm-Electrical or Pneumatic Driving Power, requires that a ventilator shall have a power failure alarm which activates an auditory signal of at least 7 seconds duration if the electrical or pneumatic power supply falls below the values specified by the manufacturer. Compliance is checked by simulating a drop below the supply power, pneumatic pressure or electrical power, required for the specified purpose of use. (International Organization for Standardization, Case Postal 56, CH-1211 Geneva 20, Switzerland)
Further to the ISO standard, the draft British Standards Institute proposal gives an example where a requirement is imposed for a visual or auditory signal of at least 7 seconds duration on power failure. (ref.: Medical Electrical Equipment-Lung Ventilators-Part 3: Particular Requirements for Emergency and Transport Ventilators (prEN 794-3), provides in Section 51.5.1 Protection Against Hazardous Output-Power Failure Alarm-Electrical or Pneumatic Driving Power, British Standards Institute, 389 Chiswick High Road, London W4 4AL)
Standards imposed in most industrialized countries will likely follow the ISO model, however to date most manufacturers of such equipment do not meet this standard. Therefore, although the risk of power failure has been recognized, there is a delay in implementing the ISO standard for audible alarms, since this standard is not mandatory. No doubt the required redesign of existing equipment to include a power fail alarm, with associated manufacturing and marketing changes will increase the cost of equipment.
Conventional supply pressure monitoring equipment may be electrically powered by batteries. The operator is required to ensure that adequate battery charge is available and unless a fail-safe low battery charge alarm is provided, the gas depletion may go unnoticed due to battery failure. Where volatile gases, such as oxygen gas, are dispensed the presence of electric power there is an inherent risk of explosion.
Automatic ventilator/resuscitators are available that are completely powered by pneumatic pressure provided by the gas supply in order to avoid the disadvantages of reliance on electric power, for example as described in U.S. Pat. No. 5,520,170 by the present inventors. For such equipment, low supply pressure effects not only the delivery of breathable gas to the patient but also the accuracy of control and monitoring circuits in the equipment itself.
It is an object of the invention to produce an in-line low supply pressure alarm device that is set off automatically when the dynamic or static supply pressure is detected below a specified minimum. Preferably, the device does not require electrical power, and is mechanically simple to ensure reliability and low production cost.
In particular it is an object of the invention to provide a low supply pressure alarm device which can be easily retrofitted to existing equipment with a minimum of operator training or equipment downtime during modification. Preferably the device may be adapted as a stand alone add on unit as well as a component included in newly manufactured equipment.